Apache Commons Net

Apache Commons Net is a Java library providing a collection of network utilities and protocol implementations. It is designed to help developers implement various network protocols within their applications.

Supported Protocols:

• Echo
• Finger
• FTP
• NNTP
• NTP
• POP3(S)
• SMTP(S)
• Telnet
• Whois

To use Apache Commons Net in your Java project, add the following dependency to your pom.xml file. Ensure you use the latest version available in the central Maven repositories.

<dependency>
  <groupId>commons-net</groupId>
  <artifactId>commons-net</artifactId>
  <version>3.13.0</version>
</dependency>

You can observe the data flowing through a Telnet session by registering an OutputStream as a spy stream. This allows you to capture what is being sent and received.

• Use _registerSpyStream(OutputStream spystream) to start spying.
• Use _stopSpyStream() to stop spying.

Note: The spy stream implementation handles character transformations (like adding \r before \n) and avoids echoing certain characters during specific negotiation states.

OutputStream spy = new FileOutputStream("session_log.txt");
telnet._registerSpyStream(spy);

To handle specific Telnet options (like echo or negotiation sub-protocols), you can register a TelnetOptionHandler using addOptionHandler(TelnetOptionHandler opthand).

If the client is already connected, the handler will immediately trigger the appropriate negotiation requests (e.g., requestWill or requestDo) based on the handler's configuration.

To remove a handler, use deleteOptionHandler(int optcode) with the corresponding option code.

The _sendAYT(Duration timeout) method sends an Are You There (AYT) sequence to the remote server and waits for a response within the specified duration. This is useful for checking if a connection is still alive without sending application-level data.

Returns true if an AYT response was received, false otherwise.

// Send AYT and wait up to 5 seconds for a response
boolean alive = telnet._sendAYT(Duration.ofSeconds(5));

You can receive notifications about Telnet option negotiation commands (such as RECEIVED_COMMAND, RECEIVED_DO, RECEIVED_DONT, or RECEIVED_WILL) by implementing the TelnetNotificationHandler interface and registering it with the client.

• Use registerNotifHandler(TelnetNotificationHandler notifhand) to attach a handler.
• Use unregisterNotifHandler() to remove it.

You can instantiate the Telnet class with a specific terminal type (e.g., "VT100") to be negotiated during the session. This is useful when the remote server requires specific terminal capabilities.

Note: The default port for Telnet is 23.

// Initialize with a specific terminal type
Telnet telnet = new Telnet("VT100");